Microsoft Security Essentials. Real-time antivirus protection (i.e. install and forget). Free.
Malware Bytes. Trojan/BotNet scanner (i.e. only runs when you launch it). Free. (And broadly considered the best secondary protection tool)
Note: Malware Bytes use CNet as a mirror which is unfortunate, because CNet is replete with adverts that contain – or are nothing more – than big “DOWNLOAD!” buttons. Watch where you click, please?
Another day, another friend I had to batter for using Avast. Ugh. Malware Bytes found 17 infections with a quick scan, removed ~200 infections with a full scan from safe mode.
He’d just renewed his Avast license too.
If nothing else, download a copy of Malware Bytes to someplace safe – a USB stick or a CD. Truly nasty infections can actually prevent you being able to update your AV scanner or download/install a new one.
Whatever AV you’re going to go with, please, pretty please, download a copy of Malware Bytes onto a USB stick or a CD and file it away for safe keeping.